1. Field of the Invention
This invention describes a new solution for obtaining user controlled high reflection factors using wideband load pull microwave tuners.
2. Description of the Prior Art
Microwave testing of low noise and high power semiconductor devices, such as transistors, require impedance generation microwave tuners. These tuners create a controlled reflection factor which is presented to the device under test (DUT) by means of test fixtures and other microwave transmission media. Modern microwave transistors, especially low noise and high power devices have very low or very high internal impedances at their input or output ports. In order to match those impedances using microwave impedance transformers, commonly called “microwave tuners” in the field, these tuners are required to be able to generate such very low or very high impedances themselves. Very low or very high impedances correspond invariably to very high reflection factors, close to 1. Such high reflection factors can be obtained actually using pre-matching networks and pre-matching tuners. These pre-matching tuners employ a pair of independently abjustable RF probes (slugs), which generate high reflection as a combination of two individual medium size reflections through vector addition of the reflection vectors [Prematching tuners for very high SWR and power and load pull measurements; Microwave Journal, January 2000, pages 176 ff.].
The solution of using pre-matching tuners in order to generate very high reflection factors has certain advantages but has also three shortcomings:
1. It can, when pre-matching is activated, allow very high reflection factors to be generated only in a small area of the Smith Chart close to and around the original pre-matching vector. Most other areas of the Smith Chart are not attained by the tuner and require re-adjustment of the pre-matching slug (FIGS. 4 and 7).
2. The size of pre-matching tuners is twice as large and up to 75% heavier than a normal single slug tuner, because each of both slugs has to be able to move independently over λ/2 to cover 360° reflection factor phase (FIG. 2)
3. The zero tuning position of the tuning section of the tuner is not 50Ω, since the pre-matching slug remains inserted in the airline (slabline) (FIG. 7)
4. Calibration time of pre-matching tuners is at least twice as long as normal tuners, if an optimized calibration technique is used, or much longer otherwise, because the tuner has to be calibrated for at least twice the number of positions, independently for each of the pre-matching and tuning sections.
The problem of high reflection factor tuning, required for Load Pull and Noise testing of high power and low noise transistors, using automatic or manual microwave tuners has been addressed up to date in different manners as follows:
1. Using (pre-matching) transforming networks (70,71) on the test fixtures (62) (FIG. 1) or “on-chip”. This allows a static pre-matching, in which the characteristic impedance of the test system is transformed to be close to the conjugate complex of the internal impedance of the DUT (device under test, 63), thus making further tuning by the external controllable automatic or manual tuners of the test setup, connected to the ports (66,67) of the test fixture (62), easier and more accurate. This technique has been used for long time in RF technology, but has the disadvantage of not being able to cover a significant frequency bandwidth simultaneously or match a variety of DUT's (63) using the same transforming networks (70,71), which said networks must be re-designed and manufactured for other frequencies and DUT's with different internal impedance.
2. Using Pre-matching tuners (FIG. 2). These devices have the capability of generating very high reflection factors using two or more RF slugs (6,7) in series and positioning them in such a manner that a first RF slug (pre-matching slug, 7) is positioned inside the airline (1) such as to generate a reflection factor (32) close to the conjugate complex of the DUT's internal impedance. Then the second RF slug (tuning slug, 6) can tune easier and more accurately around the DUT's conjugate complex internal impedance (34), on a circle (33, 72). The theory and the experimental behavior behind this approach is basically the same as in case 1, i.e. adapting the characteristic impedance of the test system to the conjugate complex internal impedance of the DUT before proceeding to the actual tuning. This method has the advantage (compared with case 1) of being flexible, i.e. by adjusting the position and depth of the first RF slug (7), we can determine the actual position on the Smith Chart (30), to be close to the conjugate complex of the actual DUT internal impedance for various DUT's, frequencies and test conditions, without having to re-design the pre-matching networks as is the case in FIG. 1. As stated before, the disadvantage of such pre-matching tuners (4) is the fact that they cannot cover the whole area of the Smith Chart, when pre-matching is activated (33), FIG. 7, and they cannot reach very high reflection factors when pre-matching is not activated (32).